1. Field of the invention
The present invention relates to a moisture sensitive material comprising a crosslinked polymer film, which detects variation of humidity in the atmosphere with a variation in the dielectric constant of the crosslinked polymer film.
2. Description of the prior art
As a moisture sensitive material wherein an electrical resistance or an electrical capacity varies depending upon a variation of humidity or water vapor in the atmosphere, there have been, for example, a moisture sensitive material having a sintered body of metal oxides such as iron oxide (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4), tin oxide (SnO.sub.2), etc., or a metal oxide film; a moisture sensitive material having a hydrophilic polymer film or a polyelectrolyte; a moisture sensitive material having an electrolyte salt such as lithium chloride (LiCl); and a moisture sensitive material having a hygroscopic resin or polymer film in which conductive particles or fibers such as carbon are dispersed.
While a moisture sensor containing a metal oxide film or a hydrophilic polymer film has generally a wide moisture-sensitivity range, its resistance varies exponentially, responding to relative humidity in the atmosphere. A moisture sensor containing a metal oxide has an excellent heat resistance and responds rapidly, but it has a high temperature resistance coefficient. Especially, moisture sensors having a sintered body of metal oxides are inferior in reproducibility and/or interchangeability of the moisture sensitive characteristic thereof because the moisture sensitive characteristic depends upon the constituents of the sensor to a great extent. A moisture sensor having an electrolyte salt such as lithium chloride detects only humidity in a narrow range and if it is allowed to stand in a high humidity atmosphere for a long period of time, the electrolyte salt therein is eluted or diluted resulting in deterioration of the moisture sensitive characteristic of the sensor, and accordingly it cannot be used for determination of high humidity A moisture sensor having a hygroscopic resin or the like, in which conductive particles or fibers are dispersed, cannot detect a humidity in a wide range because it exhibits a steep variation of the resistance thereof in a high humidity atmosphere, while it is not sensitive to low humidity. Also a moisture sensor having a hydrophilic polymer film or a polyelectrolyte film is inferior in humidity resistance, water resistance and durability, while it is advantageous in that it operates in a wide moisture sensitive range, has a rapid moisture sensitive response, a simple structure, and is easily produced at low cost.